The present invention relates generally to frequency demodulators, and particularly to a phase tracked loop (PTL) frequency demodulator wherein a feedback control signal is used to shift the frequency of the resonant peak of a filter to impart a constant phase shift to the input frequency-modulated signal over the range of the instantaneous frequencies thereof; phase comparison between the phase-shifted signal and a non-phase shifted FM signal derives an output signal.
In a CD-4 or similar system wherein frequency-modulated signals are carried on neighboring transmission channels, interference may occur between such signals through crosstalk paths generating unwanted signals which fall within the bandwidth of the channels. In order to achieve elimination of the unwanted signals, or noise, U.S. Pat. No. 4,066,972 assigned to the assignee of the present invention, discloses a phase tracked loop frequency demodulator in which the center frequency of a bandpass filter is varied in response to a feedback control signal so as to correspond to the instantaneous frequencies of a received frequency-modulated signal which passes through the filter. The received input signal is concurrently applied to a differentiator so that the differentiated output has a phase difference of 90.degree. with respect to the input signal. The differentiator output tends to vary in amplitude as a function of frequency due to the frequency response characteristic of the differentiator. A limiter is provided to compensate for the amplitude variations to apply a constant amplitude, phase shifted signal to a comparator for comparison with the output from the bandpass filter to provide demodulation. This phase tracked loop control clearly discriminates against unwanted signals, since the loop is always in tune to the instantaneous frequencies of the wanted signal.
However, because of the circuit configuration of the circuit elements such as the variable frequency bandpass filter, differentiator and limiter, it is difficult to design a frequency demodulator which operates satisfactorily in the frequency range of FM radio and television systems without adding complexity and an increased cost.